The invention generally relates to a self-elongating or length-changing actuators such as an electrorestrictive, magnetorestrictive, piezoelectric or solid state actuator. In particular, the present invention relates to a compensator assembly for a length-changing actuator, and more particularly to an apparatus and method for hydraulically compensating a piezoelectrically actuated high-pressure fuel injector for internal combustion engines.
A known solid state actuator may include a ceramic structure whose axial length can change through the application of an operating voltage. It is believed that in typical applications, the axial length can change by, for example, approximately 0.12%. In a stacked configuration, it is believed that the change in the axial length is magnified as a function of the number of actuators in the solid-state actuator stack. Because of the nature of the solid-state actuator, it is believed that a voltage application results in an instantaneous expansion of the actuator and an instantaneous movement of any structure connected to the actuator. In the field of automotive technology, especially, in internal combustion engines, it is believed that there is a need for the precise opening and closing of an injector valve element for optimizing the spray and combustion of fuel. Therefore, in internal combustion engines, solid-state actuators are now employed for the precise opening and closing of the injector valve element.
During operation, it is believed that the components of an internal combustion engine experience significant thermal fluctuations that result in the thermal expansion or contraction of the engine components. For example, it is believed that a fuel injector assembly includes a valve body that may expand during operation due to the heat generated by the engine. Moreover, it is believed that a valve element operating within the valve body may contract due to contact with relatively cold fuel. If a solid state actuator is used for the opening and closing of an injector valve element, it is believed that the thermal fluctuations can result in valve element movements that can be characterized as an insufficient opening stroke, or an insufficient sealing stroke. It is believed that this is because of the low thermal expansion characteristics of the solid-state actuator as compared to the thermal expansion characteristics of other fuel injector or engine components. For example, it is believed that a difference in thermal expansion of the housing and actuator stack can be more than the stroke of the actuator stack. Therefore, it is believed that any contractions or expansions of a valve element can have a significant effect on fuel injector operation.
It is believed that conventional methods and apparatuses that compensate for thermal changes affecting solid state actuator operation have drawbacks in that they either only approximate the change in length, they only provide one length change compensation for the solid state actuator, or that they only accurately approximate the change in length of the solid state actuator for a narrow range of temperature changes.
It is believed that there is a need to provide thermal compensation that overcomes the drawbacks of conventional methods.
The present invention provides a fuel injector that utilizes a length-changing actuator, such as, for example, an electrorestrictive, magnetorestrictive or a solid-state actuator with a compensator assembly that compensates for thermal distortions, brinelling, wear and mounting distortions. The compensator assembly utilizes a minimal number of elastomer seals so as to reduce a slip stick effect of such seals while achieving a more compact configuration of the compensator assembly. In one preferred embodiment of the invention, the fuel injector comprises a housing having a first housing end and a second housing end extending along a longitudinal axis, the housing having an end member disposed between the first and second housing ends, an length-changing actuator disposed along the longitudinal axis, a closure member coupled to the length-changing solid-state actuator, the closure member being movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection, and a compensator assembly that moves the solid-state actuator with respect to the body in response to temperature changes. The compensator assembly includes a body having a first body end and a second body end extending along a longitudinal axis. The body has a body inner surface facing the longitudinal axis, a first piston disposed in the body proximate one of the first body end and second body end. The first piston includes a first working surface distal to a first outer surface, the outer surface cooperating with the body inner surface to define a first fluid reservoir, a second piston disposed in the body proximate the first piston, the second piston having a second outer surface distal to a second working surface that confronts the first working surface, a first sealing member coupled to the second piston and contiguous to the body inner surface, a flexible fluid barrier coupled to the first piston and the second piston, the flexible fluid barrier cooperating with the first and second working surface to define a second fluid reservoir, and a first spring member and a second spring member. Each of the first and second spring members being contiguous to the second outer surface of the second piston so as to move at least one of the first piston and the second piston along the longitudinal axis.
The present invention provides a compensator that can be used in a length-changing actuator, such as, for example, an electrorestrictive, magnetorestrictive or a solid-state actuator so as to compensate for thermal distortion, wear, brinelling and mounting distortion of an actuator that the compensator is coupled to. In a preferred embodiment, the length-changing actuator has first and second ends. The compensator comprises a body having a first body end and a second body end extending along a longitudinal axis. The body has a body inner surface facing the longitudinal axis, a first piston disposed in the body proximate one of the first body end and second body end. The first piston includes a first working surface distal to a first outer surface, the outer surface cooperating with the body inner surface to define a first fluid reservoir, a second piston disposed in the body proximate the first piston, the second piston having a second outer surface distal to a second working surface that confronts the first working surface, a first sealing member coupled to the second piston and contiguous to the body inner surface, a flexible fluid barrier coupled to the first piston and the second piston, the flexible fluid barrier cooperating with the first and second working surface to define a second fluid reservoir; and a first spring member and a second spring member, each of the first and second spring members being contiguous to the second outer surface of the second piston so as to move at least one of the first piston and the second piston along the longitudinal axis.
The present invention further provides a method of compensating for distortion of a fuel injector due to thermal distortion, brinelling, and wear and mounting distortion. In particular, the actuator includes a fuel injection valve or a fuel injector that incorporates a length-changing actuator such as, for example, an electrorestrictive, magnetorestrictive, piezoelectric or solid state actuator. A preferred embodiment of the length-changing actuator includes a solid-state actuator that actuates a closure member of the fuel injector. The fuel injector includes a housing having a first housing end and a second housing end extending along a longitudinal axis, the housing having an end member disposed between the first and second housing ends, an length-changing actuator disposed along the longitudinal axis, a closure member coupled to the length-changing actuator, and a compensator assembly that moves the length-changing actuator with respect to the housing in response to temperature changes. The compensator assembly includes a body having a first body end and a second body end extending along a longitudinal axis. The body has a body inner surface facing the longitudinal axis, a first piston disposed in the body proximate one of the first body end and second body end, the first piston cooperating with the body inner surface to define a first fluid reservoir, a second piston disposed in the body proximate the first piston, the second piston having a second outer surface distal to a second working surface that confronts the first working surface, an elastomer coupled to the second piston and contiguous to the body inner surface, and a flexible fluid barrier coupled to the first piston and the second piston, the flexible fluid barrier cooperating with the first and second working surface to define a second fluid reservoir. In a preferred embodiment, the method is achieved by confronting a surface of the first piston to an inner surface of the body so as to form a controlled clearance between the first piston and the body inner surface of the first fluid reservoir; engaging an elastomer between the working surface of the second piston and the inner surface of the body; coupling a flexible fluid barrier between the first piston and the second piston such that the second piston, the elastomer and the flexible fluid barrier form the second fluid reservoir; preloading the second piston with at least one of a first spring member and a second spring member so as to generate a hydraulic pressure in the first and second hydraulic reservoirs; and biasing the length-changing actuator with a predetermined force vector resulting from changes in the volume of hydraulic fluid disposed within the first fluid reservoir as a function of temperature.